I started Eldhelm as a Rogue. so your situation may vary but i expect some of this information is useful to All classes.
Some of the features may be hard to find so here's a list of how i get to them.
Collector deck = Market > 2 cards icon
Salvage = Market > Character sheet icon > Decks > Salvage
Stats = Market > Character sheet icon > Stats (this is where you can spend level points if you dismiss them at level up)
Combo LIst = Catalog > Combos

In the Beginning:

When you first start playing your main goal should be to collect cards. this is after all a CCG(collectable card game). The easiest way to do that is to play.

1. If you can play it. You can keep it

The random deck allows you to draw cards that range from Normal to Mythic. Generally the more rare the card..the more energy it costs to play. This means the points you get when you level should be spent on Energy first! i would suggest getting your energy to 125 or better. My first hand i drew a Death Trap. it costs 124 to cast. which i never got to play because my energy was too low..so i didn't get to keep it. Cards you draw from your Collector deck are not Duplicated. They were your cards already.

2. Keeping it.

You can only put 3 of any card in a deck until much later in the game (lv 60) when you can purchase Mentality. Any extras you get you can Salvage for gold. This process can be a PAIN. In my early days i found the Salvage all button. A nice feature but DANGEROUS. You can accidentally salvage cards you meant to keep. To avoid this.. Make SEVERAL collector decks. IF ITS IN A DECK IT CANNOT BE SALVAGED. The first one you make should be your PLAY deck with whatever cards you like. The rest will be your collection decks. One for Epic/Mythic and 2 or 3 for Elite and Rares. Normal cards i wouldn't worry about saving..they are VERY easy to replace if you need them.

3. Using it.

So you have a few cards collected..some rares and elites, maybe an epic or 2, and if you are lucky..a Mythic. Its time to work on that PLAY deck. You can choose to draw cards either from the RANDOM deck or your Collector deck(i call mine PLAY) In the early stages i used the collector deck to focus on DEFENSE. On my turn i would draw from the random deck. On their turn i would draw from my play deck, which made sure i always had a defense against whatever they were attacking with. Your strategy may differ here. You could focus on Attack or Disable or Healing. Later you might want to use a technique called Short Stack.

4. Combos and the Short Stack

Eventually you are going to want to look at the combos you can create with your cards.. Certain cards played together are much more powerful than they would be individually. you can find these combos in the catalog tab. You COULD stuff your PLAY deck with a bunch of combos and hope you draw the right cards together or you could Short Stack. Short stacking is when you Intentionally leave your PLAY deck very small. This tactic can be used to get your "perfect" first hand. Or more often, First hand + 3-5 draws. This is where Combos become VERY useful...IF you have the energy to play them. Some combos only take 2 cards. some take 5! Use your Short Stack to get those 5 card combos off. Plan out your first 2 turns and Rush or Disable your opponent.

5. Leveling.

You gain experience for winning or losing. So if your playing to gain cards...dont worry so much about winning. it might be worth drawing the fight out to get more chances to draw better cards without sitting thru loading screens. Eventually you will level up! As you level up you get some nice perks at certain levels.
A. Your hand size will go up to a maximum of 10
B. Your "buff" slot goes up to a maximum of 3
C. Your collector deck can be bigger. (mine is at 60 so far)
D. You get points to spend on your stats like Energy, HP, Might, Resistance, or Regeneration